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Genomics in Action: William J.Pavan, Ph.D.

Greener Pastures: Exploring the Genetics of Pigmentation

The black and white patterns on the hides of dairy cows —
the same patterns that conjure thoughts of a certain brand of ice cream — may also have been the inspiration behind a career in genomics. Their seemingly infinite shapes and variations — somehow not completely random — captured the imagination of a young animal sciences major at just the time he encountered the field of pigment genetics.

Forsaking dairy farms for the frontiers of genomics, William J. Pavan, Ph.D., went on to become a senior investigator in the Genetic Disease Research Branch of the National Human Genome Research Institute (NHGRI). Today, his research holds promise for understanding melanoma, the most deadly type of skin cancer, as well as an array of rare genetic disorders.

Dr. Pavan's work centers on the neural crest, a structure in the rapidly developing embryo. Cells that originate from the neural crest propagate and are programmed for important functions all around the body. Some become melanocytes, cells that produce the pigments that color skin, hair and, in animals, fur. Others develop into tissues that make up key craniofacial, nervous system and digestive tract components.

Dr. Pavan is intent on identifying all the genes that are involved in controlling the development of neural crest cells that differentiate into melanocytes. To this end, he is focusing on two main types of disorders: those that arise from an uncontrolled growth of melanocytes, including melanoma, and those characterized by an insufficiency of melanocytes.

"Our hypothesis is that the same genes and pathways are involved in both types of disorders," Dr. Pavan said. "We study these pathways to see if they are misregulated in melanoma and, if they are, whether we can intervene and modulate them." Discovering the genes or pathways that are disrupted in melanoma could open the door to better methods for diagnosis, treatment and prevention.

Dr. Pavan's career path, from large-animal field work as an undergraduate at University of Massachusetts-Amherst to his eventual study of mouse embryology at the Bethesda campus of the National Institutes of Health (NIH), had unexpected turns. "At one time, I couldn't envision dedicating my professional life to laboratory research, much less the genetics of pigmentation," he said. But he did precisely that. He earned his Ph.D. from The Johns Hopkins University in Baltimore upon completion of a combined biochemistry, molecular biology and biophysics program. He then completed a post-doctoral fellowship in mouse molecular genetics at Princeton University.

Pigmentation genetics research frequently employs the mouse model. At Princeton, Dr. Pavan investigated the biological basis underlying coat color alterations in piebald mice &mdash a strain appreciated among mice-breeding hobbyists for their striking spots and patches. His research focused on a set of gene modifiers called k-complex genes, which collectively contribute to spotting patterns. He took great satisfaction in employing new genetic and genomic approaches to address decades-old theories about these genes; he was ultimately able to map and evaluate the interactions and positions of these k-complex genes on mouse chromosomes.

Dr. Pavan's enthusiasm for the field of pigment genetics has grown over time. He arrived at NHGRI in 1993 and, since that time, has developed mouse models to understand molecular mechanisms and defects that are relevant to a range of human developmental disorders.

The cutting-edge techniques that are employed by Dr. Pavan include genetic screening, gene expression analysis and comparative genome sequencing of multiple species. Such techniques enable him to explore the genetic network of pigmentation, and to hunt for DNA elements that regulate particular genes. "I am fascinated that subtle perturbations of gene interactions can be caused by one fewer or an extra copy of a subset of genes in an otherwise normal cellular environment," he said.

In addition to high-tech methods, Dr. Pavan utilizes what he calls "classic mouse genetics," where mice are bred for detection of gene pathways and interactions. "The mice do a lot of the work for us. We selectively breed the mice, and their offspring tell us whether there is a gene interaction and the relative order of the interaction in that pathway," Dr. Pavan said.

Mice are particularly good models for studying melanocyte genetics because many strains of mice with differing coat patterns have been specially bred, with each coat pattern reflecting a different spontaneous mutation in a gene or the genes governing melanocyte development. Close examination of the physical characteristics of the mouse reveals how particular genes function; researchers simply look at coat colors and patterns.

A normal mouse has a completely solid appearance, typically black or agouti — a blending of yellow, black and brown fur; hobbyists refer to multicolored stock as "fancy mice." The piebald mice mutants studied by Dr. Pavan have a solid coat, except for certain white patches. These white patches hold the clues to understanding gene interactions.

Dr. Pavan explained that random genetic mutations occur after breeding mice for generations. "In mice, if a white spot shows up, people pull those mice to the side and breed them," he said. "You would imagine that, after hundreds of years, all of the genes that cause spotting would already be found, but we're still finding new genes in this pigmentation pathway."

The worlds of 21st century genomics and classic mouse genetics meet in a technique called mutagenesis screening. For Dr. Pavan's work, the screening starts with a mouse sensitized to have neural crest defects and fewer neural crest cells. By systematically inducing mutations using a mutagenic chemical and then screening for particular features, specific mutations can be identified that cause an amplification of the disease state. The Pavan laboratory has used a particular screen for five years to detect new genes that impact on the development of melanocytes.

The laboratory also studies how these newfound genes fit into a complex network of interrelated genes. Unlocking the puzzle of this gene hierarchy will lead to a better understanding of the roles that these genes play in disease. "We are just starting to understand this process, but our mutagenesis screen has already detected genes that have never been discovered before in this pathway," he said. "We are finding two to three new genes a year."

"These mice have white areas on their coat because of a deficiency in their neural crest development," Dr. Pavan explains. Since one gene can cause multiple effects — a phenomenon called pleiotropy — a harmless trait like white spots can often be coupled with a more serious biological disorder.

The Pavan laboratory is interested in multiple disorders of this kind. In a particular variety of mouse, the pleiotropic gene is involved with both neural crest development and limb formation; mice with this trait have extra toes. Another mouse variety has a pleiotropic gene leading to white spotting and small brain size. In this case, the gene that causes both traits is important to understanding functions of the neural crest, particularly pigmentation and central nervous system development.

As part of his interest in melanocytes, Dr. Pavan also studies the effect of the transcription factor SOX10 and its gene. This transcription factor regulates neural crest development in vertebrates and the migration of neural crest cells throughout the body. The differentiation of neural crest cells into different tissue elements depends upon the level of SOX10 expression. Too little expression of SOX10 causes the neural crest cells to die, as seen in Waardenburg syndrome. Since hair follicles and cells of the iris develop from neural crest-derived melanocytes, the melanocyte defects seen in patients with Waardenburg syndrome can cause the development of white patches in their hair, as well as lighter eye coloration or differences in the color of the left versus right eyes. The melanocyte defects also can cause devastating outcomes such as deafness, since melanocytes are found in structures of the cochlea vital in hearing. Neural crest cells also are found in the digestive tract, and when dysfunctional, cause a disorder known as Waardenburg-Hirschsprung disease.

In the course of his career, Dr. Pavan has been guided by keen observation, whether in the natural environment or enhanced by the dynamic new tools of genomes. His efforts have yielded new insights about a category of disorders that still hold many mysteries, but every discovery along the way gives him a sense of satisfaction. According to Dr. Pavan, "being the first person to see a mutation in a mouse, establish what gene is involved, and learn how it fits into the pathway with many other genes &mdash that's really exciting."